We have previously described a novel human gene related to the abl proto- oncogene, termed arg, located at chromosome position 1q24-25 expressed as two 12-kb transcripts encoding proteins that differ only in their amino termini. The comparison of the predicted amino acid sequence of arg with that of c-abl reveals important structural features common to both proteins, and defines the Abelson subfamily of cytoplasmic tyrosine kinases. The availability of arg cDNA clones has allowed exploration of the biological properties of arg by means of expression in mammalian cells as well as bacteria. In order to develop immunological reagents, segments of the arg gene were expressed in bacteria as T10 fusion proteins using an inducible expression system. The arg proteins thus synthesized were purified and injected into rabbits to raise polyclonal sera, as well as into mice to develop monoclonal arg antibodies. Arg constructs have also been engineered to express various arg proteins in mammalian cells. The initial expression vectors direct the synthesis of the normal arg protein and a gag-arg chimeric protein similar to v-abl. Analysis of the gag-arg protein when transiently expressed in COS cells revealed that it was activated with regard to its tyrosine kinase activity, and that it was phosphorylated on tyrosine in vivo. Stable transfectants expressing the gag-arg protein were obtained by transfecting 32D, a hematopoietic cell line. Work is underway to express arg proteins in NIH/3T3 cells as well. Both of these protein forms have also been expressed in bacteria to study their catalytic activities in vivo.